Secretion of HCl by the gastric oxyntic cell is a highly specialized process involving coordinated 1) secretagogue-receptor interaction, 2) cytoplasmic activation, 3) recruitment of cytoplasmic membranes into the apical cell surface, and 4) operation of highly energetic transport pathways. The developing system offers a unique opportunity to study the molecular features and limitations in the series of events that underlie HCl secretion. Morphological differentiation of the oxyntic cell occurs rather rapidly, indicating defined periods of high synthesis for particular elements of the secretory apparatus. We propose to evaluate the ontogeny of individual components of the secretory apparatus along with the ultrasturctural development of the oxyntic cell and the ability to transport HCl. The development of the H+ pump enzyme, H,K-ATPase, will be studied and correlated with the pattern of tubulovesicle synthesis within the cell. Specific enzyme activity, total enzyme quantity and relative rates of synthesis of the pump enzyme will be determined. H+ transport activity of isolated membrane fractins deriving from cytoplasmic tubulovesicles will be studied and compared with H+, K+ and C1- transport with membranes that have been transformed (by fusion) into the apical cell surface. The ontogeny of oxyntic cell secretagogue-receptor activity will be established and compared with the appearance of intracellular second messenger activating systems as well as the capacity of the various transport pathways. Pathways for membrane synthesis during cytodifferentiation and membrane flow during the secretory cycle will be established. Analysis of mRNA durig oxyntic cell differentiation will seek to identify stages where specifically coded messages, such as that for H,K-ATPase synthesis, are in high abundance. These studies will provide a comprehensive picture of the chronology for expression of surface receptors. intracellular activators, transport enxymes, and ion channels that constitute the integrated secretory machinery of the oxyntic cell. The ontogenic appearance of specific proteins and transport pathways will also serve as identifying markers and help to establish their relative importance in the process of HCl secretion.